This invention relates to a voltage detector which employs an optical modulator to detect voltages with a time resolution of the order of picoseconds.
A voltage detector for detecting voltages with an optical modulator is well known in the art. FIG. 5 is a block diagram of a voltage detector of this type disclosed by U.S. Pat. No. 4,446,425.
In the voltage detector of FIG. 5, a pulse light source 50 outputs two beams of short light pulses of the order of 120 femtoseconds (10.sup.-15 seconds) repeatedly. One of the short light pulses is applied through a chopper 51 and a variable delay unit 52 to an object under measurement 53 such as a photo-electric switch, and the other beam is applied directly to an optical modulator 40. The optical modulator 40 comprises a polarizer 55, a Pockels cell 54, a phase compensator 56, and an analyzer 57. The optical modulator 40 operates on the principle that the incident short light pulse is modulated by the voltage of the object under measurement 53, to obtain a voltage waveform represented by optical intensity. This will be described in more detail. A voltage to be detected which is outputted by the object 53 in synchronization with the short light pulse is applied to the Pockels cell 54 in the optical modulator 40, while a predetermined polarization component of the short light pulse outputted by the pulse light source is extracted by the polarizer 55 and is applied to the Pockels cell 54. The Pockels cell 54 is made up of an electro-optical material such as LiNbO.sub.3 or LiTaO.sub.3 with a refractive index which is changed by a voltage applied thereto. Due to the nature of the electro-optical material, the short light pulse applied to the Pockels cells is changed in polarization by the voltage of the object under measurement 53; that is, the short light pulse is modulated by the voltage, and output as an emergent light which is applied through the phase compensator 56 to the analyzer 57. The analyzer 57 extracts two orthogonal polarization components from the emergent light provided by the phase compensator, and applies modulated optical intensity signals as the outputs of the optical modulator, to photodetectors 58 and 59, respectively. The photodetectors 58 and 59 detect the optical intensities of the two polarization components, respectively. In a differential amplifier 60, the output signals of the photodetectors 58 and 59 are subjected to differential amplification. The output of the differential amplifier 60 is applied through a lock-in amplifier 61 and an averaging unit 62 to a display unit 63.
The variable delay unit 52 operates to gradually delay the timing of providing the voltage from the object under measurement 53, in order to establish a sampling point. The lock-in amplifier 61 operates to fetch the output of the differential amplifier 60 in synchronization with the chopper 51 to remove noise components. The averaging unit 62 operates to average the output of the lock-in amplifier 61.
In the voltage detector as described above, the photodetectors 58 and 59 have a slow response speed, and therefore, the optical intensity signals of the emergent light from the analyzer 57 have a time width of the order of ten nanoseconds in the photodetectors 58 and 59 as shown in FIG. 7(c). Accordingly, it is essential that the repetitive period of provision of a voltage from the object under measurement 53 is set to higher than ten nanoseconds.
The case where the waveform of a voltage provided by the object under measurement 53 has a time width of 100 picoseconds as shown in FIG. 6 and is detected with a time resolution of one picosecond will now be discussed. In this case, the variable delay unit 52 is initialized at the sampling point S.sub.1, and the optical intensity at the sampling point S.sub.1 is sampled. The sampling operation is repeatedly carried out with a pulse train having a repetitive period of the order of 10 nsec. (nanoseconds) as indicated by a solid line in FIG. 7(a), and the sampled data is averaged by the averaging unit 62. Then, the variable delay unit 52 is operated to shift the sampling point S.sub.1 to the following sampling point S.sub.2 which is shifted from the sampling point S.sub.2 by 1 psec. (picosecond), and the optical intensity at the sampling point S.sub.2 is obtained in the same manner. That is, the variable delay unit 52 is operated one hundred times in the above-described way to obtain one hundred sampling point from S.sub.1 through S.sub.100, and the optical intensities at the sampling points from S.sub.1 through S.sub.100 are repeatedly obtained and averaged, so that the voltage waveform is detected by sampling. At each of the sampling points from S.sub.1 through S.sub.100, the optical intensity signal has a time width of the order of 10 nsec., however, since the sampling timing occurs at intervals of 1 psec., the voltage waveform can be detected with a time resolution of 1 psec.
As described above, in accordance with the conventional voltage detector, a voltage waveform having a time width of 100 psec. is detected with a resolution of 1 psec., and therefore the variable delay unit 52 must be operated one hundred times. That is, the conventional voltage detector has the disadvantage that it is impossible to achieve the detection in a short period of time. If the variable delay unit 52 must be moved for instance 0.3 mm. for a delay of 1 psec., then the variable delay unit 52 must be totally moved 30 mm for sampling when operated one hundred times; that is, the variable delay unit is moved a relatively long distance. This limits the miniaturization and the structural simplification of the variable delay unit 52.